Macrophage activity is an important indicator of the normal function of the major organs of the reticuloendothelial system (e.g. liver, spleen, lymph nodes, and bone marrow). In disease, important abnormal sites of macrophage activity include tumors, inflammatory lesions, heart attack lesions, stroke lesions, and major vessel atherosclerotic plaque. Macrophage activity is often viewed by imaging uptake of nanoparticles. However, there is a lack of nanoparticles for imaging macrophage function, especially with radioactivity-based imaging modalities like positron emission tomography (PET) or single-photon emission computed tomography (SPECT).
Radiolabeled magnetic NPs can be imaged by PET or SPECT as well as by magnetic resonance imaging (MRI). The most common method of obtaining radiolabeled magnetic NPs is by the covalent attachment of chelates to the NP surface. However, labeled NPs can also be obtained by the adsorption of radiometal ions to the NP surface or by the addition of a radiometal during NP synthesis. Each method, however, has limitations. A chelator on the surface of a metal-bearing NP can be “poisoned” by small amounts of extractable metal, especially if storage of the chelate-bearing NP prior to radiolabeling is attempted. Moreover, the adsorption of metal ions to the oxide surface may not be irreversible, especially in complex biological media, and is limited to those metals that bind to the NP surface. Radiometal addition during synthesis requires the use of relatively long-lived isotopes, with purification of the radioactive NP and disposal of now radioactive liquid waste. In addition, the starting NPs used for radiolabeling typically have unknown toxicological properties and/or a lack of history of clinical use.
One iron oxide nanoparticle called ferumoxytol (e.g., FERAHEME® (FH), available from AMAG Pharmaceuticals, Inc., Lexington, Mass.), approved for the treatment of iron anemia, can be used to image macrophage function. The iron of ferumoxytol is superparamagnetic and alters magnetic resonance (MR) signals. The structure of FH is shown in FIG. 1a. Ferumoxytol includes a core of 5874 iron atoms present as a super-paramagnetic iron oxide and a relatively thick coating (approximately 10 nm) of carboxy-methyldextran (CMD). The overall diameter of a ferumoxytol nanoparticle is about 25 nm.
The FH NP formula is Fe5874O8752:C11719H18682O9933Na414. As a radiolabeling substrate, FH NP is advantageous because of its wide availability, well-understood pathways of degradation, and demonstrated clinical safety at high anemia-treating iron doses (relative to the lower iron doses that might be used in PET or SPECT imaging). FH may be used to treat iron anemia by bolus injections of roughly 500 mg of iron. FH is also used off-label to image infiltrative macrophages by MR in many clinical settings.
Although ferumoxytol is indicated for the treatment of iron anemia, the iron oxide core is superparamagnetic, and it is moderately detectable by MRI. Thus, ferumoxytol has been widely used as an MRI contrast agent for macrophages in various imaging clinical trials. Most clinical trials use ferumoxytol at or around the high dose toxicity limit, and many give poor or marginal results. Trials using ferumoxytol have been conducted in areas including neuroinflammatory states, CNS neoplasms, imaging myocardial infarction, imaging the progression of type I diabetes, imaging the metastatic status of lymph nodes, imaging central nervous system (CNS) function, and the like.
Use of ferumoxytol as an MRI agent has several limitations. First, high doses and small MR signal intensity changes are characteristic of some tissues, and the dose of ferumoxytol often cannot be increased due to iron toxicity. Some tissues have weak macrophage activity that cannot be visualized when using ferumoxytol and MRI. Moreover, MRI cannot quantify the amount of ferumoxytol in a tissue. On the other hand, PET imaging gives quantitative data on nanoparticle tissue concentrations. Moreover, FH/MRI techniques often require the administration of anemia-treating doses of 400-500 mgs of Fe to non-anemic individuals. It is estimated that heat-induced radiolabelling (HIR) FH for imaging macrophages by SPECT or PET could involve iron doses of only about 5-50 mg/person. In addition the PET imaging of radiolabeled NPs offers higher detectability of tissue NP concentrations and the ability to quantify of 89Zr concentrations in tissues or organs. Thus, the replacement of FH/MRI with 89Zr-FH/PET or 111In/SPECT might offer advantages for imaging macrophage infiltration in various pathologies.
There are also significant barriers to developing new macrophage imaging agents. First, there are often high regulatory barriers and/or costs associated with the de novo development of clinical imaging agents. Second, current radiolabeling methods are selective for one radioactive metal or another, which limits the applications, markets, and utility of a radiolabeled nanoparticle for macrophage imaging. Third, it is desirable that the radiolabeled nanoparticle be detectable by at least two imaging modalities. This allows multimodal imaging of the same patient to enhance the quality of information obtained.
Thus, there exists a need for a simple yet general radiolabeling method for iron oxide nanoparticles that minimizes the regulatory costs associated with the clinical translation of a radiolabeled nanoparticle. It is desirable that the macrophage imaging agent meeting these conditions be detectable by at least two modalities (e.g., MR and a radioactive imaging method such as PET, SPECT, or Cerenkov imaging).